With the advent of induced draft combustion apparatus for furnaces and the increased cost of fuel, in particular natural gas, continued research and development is being made to provide a control system for near stoichiometric control or fuel/air ratio control of the combustion of fuel in furnaces. In furnaces where the amount of air supplied to the combustion chamber by the induced draft blower can be changed by many environmental situations or from application to application for a given furnace design, the need of a control system for adjusting the fuel/air ratio with the changes in air flow has greatly increased.
The present invention makes use of a single pressure transducer apparatus for providing an output signal indicative of the pressure to which the transducer is exposed. By alternately connecting the transducer so that it responds to the pressure of the fuel to the burner of the furnace or to the pressure of the fuel plus the suction of the induced draft blower, an output signal is available to a controller. The fuel control valve is modulatingly controlled to maintain combustion in the furnace with a predetermined fuel/air ratio for maximum efficiency. Specifically, the single transducer means is alternately connected by means of a valve to respond to the difference between gas pressure and atmospheric pressure or blower pressure to obtain the signal having two levels depending upon the sensed pressures. By means of a summing network these pressure signals can effectively control a modulating valve and thus the fuel control to the burner to maintain the proper fuel/air ratio as the supplied quantity of combustion air changes.